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( 




Bureau of Mines Information Circular/1985 



Design and Operation of Four Prototype 
Fire Detection Systems in Noncoal 
Underground Mines 

By William H. Pomroy and Robert E. Helmbrecht 




UNITED STATES DEPARTMENT OF THE INTERIOR 



.751 

%INES 75TH AV"!^ 



Information Circular 9030 

1/ 



Design and Operation of Four Prototype 
Fire Detection Systems in Noncoal 
Underground Mines 

By William H. Pomroy and Robert E. Helmbrecht 




UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Model, Secretary 

BUREAU OF MINES 
Robert C. Morton, Director 







^yw^. 



Gfo^-b 



Library of Congress Cataloging in Publication Data; 



Pomroy, William H 

Design and operation of four prototype fire detection systems in 
noncoal underground mines. 

(Information circular ; 9030) 

Bibliography: p. 18. 

Supt. of Docs, no.; 1 28.27:9030. 

1. Mine fires. 2. Fire detectors. I. Helmbrecht, Robert. II. Se- 
ries: Information circular (United States. Bureau of Mines) ; 9030. 



fTN315] 622s [622\8] 84-600372 



CONTENTS 



Page 



Abstract I 

Introduction 2 

Fire detection system for a timbered salt mine shaft 2 

Smoke detection system for a multilevel copper mine 6 

Spontaneous combustion fire warning system for a deep silver mine 10 

Computerized fire detection system for an underground trona mine 12 

Conclusions 17 

References 18 

Appendix 19 

ILLUSTRATIONS 

1. Layout of shaft fire detection system 3 

2. Thermistor strip shaft fire detector and junction box attached to shaft 

t imbe r 4 

3. System control panel in headframe building 4 

4. Shaft fire detection system annunciator panel in hoist house 4 

5. Installation of thermistor strip shaft fire detector from a canopy above 

the man cage 4 

6. Annunciator panel during system test showing alarms and digital hotspot 

indicator 5 

7. Mineworthy ionization-type combustion particle (smoke) detector 6 

8. Digital telemetry module mounted inside detector cap 7 

9. Installation of ionization-type combustion particle (smoke) detector at 

the 500-250 ramps 7 

10. Mine map on video display showing location and output of detectors in the 

collar-decline area, the oxide extraction area, and the sulfide extrac- 
tion area 8 

11. Video display showing three-key function commands 9 

12. Layout of major elements of spontaneous combustion detection system 9 

13. Spontaneous fire warning system detection instruments 10 

14. Draw tube supplying mine air from fan cowling to detection instruments... 11 

15. Strip chart recorder in guard shack 12 

16. Telemetry interface modules adjacent to detection instruments 12 

17. Telemetry interface modules in guard shack 13 

18. Typical chart recording showing elevated CO levels following end-of-shift 

blasts 13 

19. Layout of major elements of trona mine fire detection system 14 

20. Added remote unit A attached to existing remote unit 14 

21. Master station 15 

22. Thermistor line-type detection at conveyor drive 16 

23. Carbon monoxide and smoke detection instruments located downstream of the 

conveyor drive 16 

24. Ultraviolet flame detector in grease niche area 17 

TABLE 

A-1. Temperature versus resistance for thermistor strip shaft fire detector... 19 





UNIT OF MEASURE 


ABBREVIATIONS USED 


IN THIS REPORT 


A 


angstrom 


min 


minute 


c/s 


cycles per second 


nA 


nanoampere 


"C 


degree Celsius 


Q 


ohm 


eV 


electron volt 


ppm 


parts per million 


ft 


foot 


pet 


percent 


°F 


degree Farenheit 


V 


volt 


h 


hour 


V ac 


volt, alternating current 


in 


inch 


V dc 


volt, direct current 


m/s 


meters per second 


W 


watt 


mA 


milliampere 


yr 


year 


mCi 


millicurie 







DESIGN AND OPERATION OF FOUR PROTOTYPE FIRE DETECTION 
SYSTEMS IN NONCOAL UNDERGROUND MINES 

By V/illiam H. Pomroy ^ and Robert E. Helmbrecht^ 



ABSTRACT 

Fires in underground metal and nontnetal mines pose a threat to the 
safety of underground miners and to the productive capacity of this Na- 
tion's mines. Contaminated air (smoke, carbon monoxide, and other 
products of combustion) is the primary life safety hazard created by a 
mine fire. The most reliable defense against the hazard posed by the 
rapid spread of contaminated air underground is early warning fire de- 
tection and rapid evacuation. This Bureau of Mines report describes the 
design and operation of four prototype early warning fire detection sys- 
tems, for underground noncoal mines, presently undergoing prolonged in- 
mine testing by the Bureau. The systems are described within the con- 
text of the underground mine environment. 



^Supervisory mining engineer. 
'^Stuaent trainee (electrical engineering). 
Twin Cities Research Center, Bureau of Mines, Minneapolis, MN. 



INTRODUCTION 



Fires in underground noncoal mines pose 
a serious threat to the safety of un- 
derground miners and to the productive 
capacity of this Nation's mines. The 
potential for loss of life and interrup- 
tions to production due to fires is often 
underestimated, even within the mining 
community and despite documented fire 
statistics to the contrary. 

This misconception was addressed by a 
senior Mine Safety and Health Administra- 
tion official (J_) : -^ 

Over the years there has devel- 
oped a generally accepted opinion 
that major disasters from fires 
do not occur in noncoal mines. 
The Sunshine Mine disaster should 
have erased that opinion. Fires 
in mines are not unusual. We have 
a continuing history of fires in 
North American mines. 

Historically, timber has been the 
primary source of fuel in the major 
mine fires, and mine operators at 
properties that do not use timber 
for ground support tend to believe 
they do not have a potential fire 
problem. Let us point out now that 
there are other sources of fuel 
for combustion. So long as inter- 
nal combustion engines, electrical 
equipment, lubricant storage, fuel 
stores, combustible hydraulic sys- 
tems, warehousing of combustible 
solvents, combustible ventilation 
tubing, and timber support are nec- 
essary to our mining systems, the 
potential of mine fires remains. 
Indeed, since 1965, over 150 fires 
accounting for 119 fatalities have been 
reported in underground noncoal mines 
(_2 ) . Countless millions of dollars have 
been spent on rescue and recovery, equip- 
ment repair and replacement, and mine 



rehabilitation. In addition, mines shut 
down by fires have been forced to forego 
hundreds of millions of tons of mineral 
production. 

Contaminated air is the primary life 
safety hazard in an underground mine 
fire, accounting for over 78 pet of mine 
fire deaths since 1945 (3^). Ventilation 
streams carry smoke, carbon monoxide, and 
other toxic fire gases to areas of the 
mine remote from the fire itself, thereby 
exposing miners who may be widely scat- 
tered throughout the working to toxic 
fire gases. 

One means of defense against the hazard 
posed by the rapid spread of contaminated 
air is early warning fire detection and 
rapid evacuation. The data show that 
fires detected within 15 min of develop- 
ment result in little or no damage to the 
mine in 73 pet of all cases (2). Effec- 
tive, reliable detection systems, capable 
of detecting fires at their early, or 
even incipient stages, can significantly 
improve mine safety by ensuring adequate 
time for mine personnel to follow appro- 
priate emergency procedures. Since 1968, 
however, only about one-third of reported 
fires were detected within 15 min ( 2_) . 

This Bureau of Mines report describes 
the design and operation of four proto- 
type fire detection systems developed for 
noncoal underground mines. Results of 
prolonged in-mine testing and system de- 
sign refinements will be presented in a 
subsequent report. 

The body of this report contains four 
sections, each describing oae of the pro- 
totype detection systems. The detection 
instruments employed in this research 
program, several of which are common to 
more than one detection system, are de- 
scribed in detail in the appendix. 



FIRE DETECTION SYSTEM FOR A TIMBERED SALT MINE SHAFT 



Although all mine fires present the 
potential for disaster, fires in mine 
shafts are particularly hazardous because 
rapid and safe egress of miners and prop- 

-^Underlined numbers in parentheses re- 
fer to items in the list of references 
preceding the appendix. 



er mine ventilation can be seriously im- 
paired by shaft fires. A recent study of 
mine fires shows that since 1950, about 
9.7 pet of all mine fires occurred in 
shafts, but about 17.6 pet of all mine 

fire fatalities were attributed to shaft 
fires (2). 



Carbon monoxide, carbon dioxide, and 
submicron particulate detectors used sep- 
arately or in combination, have been used 
successfully to provide early warning of 
shaft fires (4^) , however these detectors 
require occasional maintenance and cali- 
bration. Industrial-grade thermal fire 
detection devices are generally charac- 
terized by high reliability and durabil- 
ity but low maintenance requirements, 
even when used under the harshest condi- 
tions. Clearly, these attributes are de- 
sirable for mining applications. 

One limiting feature of thermal detec- 
tors is that they rely on convected ther- 
mal energy for response. The distance 
between the detector and the fire, the 
relative spatial orientation and place- 
ment of the detector relative to the 
fire, and local air currents profoundly 
affect detector performance. Thus, in 
order to provide for large area coverage, 
numerous closely spaced detectors are 
required. A common example of thermal 
detection in the mining industry is the 
typical conveyor belt fire detection 
system mandated for underground coal 
mines (5^). Spot-type, thermal detectors, 
spaced at 125-ft intervals along the en- 
try provide early warning of a belt fire. 

A string of spot-type detectors arrayed 
in a similar manner in a mine shaft is a 
feasible approach to shaft fire detec- 
tion, however, the use of a line-type de- 
vice would offer superior performance. A 
line-type device senses the heat from a 
fire at any point along its length. It 
can be thought of as spot-type detection 
in the limiting case where the distance 
between adjoining detectors equals zero. 

Limited success has been achieved using 
fusible contact line-type thermal sensors 
in shafts (^) . However, fusible contact 
line-type sensors are subject to occa- 
sional false alarms and considerable ef- 
fort may be required to restore the de- 
tector to proper operation following an 
alarm, especially if the contact occurs 
in a section of the shaft for which ac- 
cess is difficult. An alternative to the 
fusible contact detector is thermistor 
strip. 

A prototype thermistor strip fire de- 
tection system for mine shafts was devel- 
oped by the Bureau and installed in the 



1,200-ft main production shaft of a salt 
mine in Detroit, MI. The system was in- 
stalled along the entire length of the 
shaft (fig. 1). The detector is de- 
scribed in detail in the appendix. 

The system provides two alarm tempera- 
ture settings, permitting a prealarm at a 
lower temperature and an alarm at a high- 
er temperature. The system also includes 
a hotspot indicator that pinpoints the 
location of the overheated area and pro- 
vides digital readouts of the distance 
between the shaft collar and the hotspot. 

The system has three main subsystems: 
the sensor element in the shaft, the 
system control panel in the headframe- 
crusher building, and an alarm annun- 
ciator panel in the hoist house. The de- 
tector is positioned roughly in the cen- 
ter of the two-compartment shaft. It is 
attached at each timber set with special 
mounting brackets, thereby providing sup- 
port for the detector at approximately 
4-ft intervals. The detector is divided 
into two zones, the upper zone and the 
lower zone, with the two thermistor ca- 
bles joined in a junction box at the 
shaft midpoint (fig. 2). The system con- 
trol panel (fig. 3) contains all control 
circuits, backup power supply, and means 
for calibrating and troubleshooting the 
system. The annunciator panel (fig. 4), 
within sight of the hoist operator, pro- 
vides a green lamp indicating normal 
system operation, visual and audible 
indication of prealarm and alarm condi- 
tions, and a digital display of the hot- 
spot location. 

Headframe and crusher building 

System control panel 
Ho ist hou se ./^ y^ 

Surface 




FIGURE 1. - Layout of shaft fire detection system. 










FIGURE 2. - Thermistor strip shaft fire detector 
and junction box attached to shaft timber. 



V 







5?%^' 



'^<¥«9!«Si5 




sj/;'-.'''. 







^^.' .'^' 



FIGURE 3. - System control panel in hecdframe 
building. 




FIGURE 4. - Shaft fire detection system annun- 
ciator panel in hoist house. 




FIGURE 5. - Installation of thermistor strip shaft 
fire detector from a canopy above the man cage. 
Cage doors are open for illustration purposes only. 



The system was completely installed by 
a three-person crew over a 4-day period 
in April 1982. The detector was in- 
stalled from a platform above the skip 
(fig. 5) by interconnecting ten 120-ft 
detector segments end to end. Because 
this shaft is the main mine exhaust, the 
air is laden with salt. This highly cor- 
rosive atmosphere is detrimental to the 
operation of electrical systems, necessi- 
tating great care in hermetically sealing 
each detector segment interconnection 
with a silicone adhesive-sealant. All 
external parts of the detector wire and 
connections are stainless steel which has 
been further protected with a corrosion- 
resistant Teflon'^ fluorocarbon polymer 
jacket. The control panel and annuncia- 
tor panel are housed in dust-tight enclo- 
sures. Following installation, the sys- 
tem was functionally tested. At a known 
elevation in the shaft, a propane torch 
was used to heat a section of the detec- 
tion cable. The prealarm and alarm func- 
tions operated properly and the hotspot 
indicator displayed the correct elevation 
(fig. 6). 

The system was operated continuously 
from April 1982 until June 1983 without 
hardware failure. Once during that peri- 
od, a lightning strike at the headframe 
structure caused a momentary alarm, how- 
ever, the system returned to the normal 
operating mode without further incident. 
These preliminary test results are sig- 
nificant because they indicate that the 
hardware and installation precautions are 
suitable for this worst case corrosive 
environment . 

In June 1983, mine officials reported 
a system failure. A technician was 
dispatched to the mine to inspect the 
system, determine what repairs and/or 
equipment replacement were required, and 
recommend system modifications (if any) 
needed to avoid future similar problems. 

The technician found that four 120-ft 
detector segments in the shaft had been 
ripped from their mountings by an object 

^Reference to sjjecific products does 
not imply endorsement by the Bureau of 
Mines . 




FIGURE 6. - Annunciator panel during system 
test showing alarms and digital hotspot indicator. 

protruding from the skip. All four seg- 
ments needed to be replaced. As a pre- 
caution to prevent further damage in the 
future, it was recommended that a 1/8- 
in-diam stainless steel messenger cable 
be installed in the shaft parallel to the 
detector and flush with the timber sets. 
The detector could then be removed from 
the mounting brackets and attached to the 
messenger. This mounting arrangement 
would (1) draw the detector closer to the 
timber sets so that it will be less like- 
ly to become entangled with objects pro- 
truding from the skip, (2) permit the 
detector to be secured at intervals 
closer than the 4-ft spacing of the tim- 
ber sets, and (3) provide greater overall 
strength to the installation because the 
stainless steel messenger cable is much 
stronger than the detector. 

Repairs to the damaged portion of the 
detector have been delayed because of a 
production shutdown at the mine. Re- 
placement of the damaged segments and 
installation of the messenger will be 
effected and testing continued upon re- 
opening of the mine. 



SMOKE DETECTION SYSTEM FOR A MULTILEVEL COPPER MINE 



One of the earliest products of In- 
cipient combustion Is submlcrometer sized 
particulates, or smoke (6^). Smoke de- 
tection systems that are capable of re- 
liably detecting these particulates are 
extremely valuable because fires can be 
detected before they reach the flam- 
ing combustion stage. With the aid of 
such systems, emergency procedures, such 
as personnel evacuation and fire fight- 
ing efforts, can be undertaken at the 
earliest opportunity, often before the 
fire poses a direct threat. A complete 
prototype smoke detection system was 
designed, fabricated, and Installed In 
an Arizona underground copper mine for 
prolonged testing and evaluation. 

The system consists of 10 detection 
Instruments (fig. 7). Each detector is 
equipped with a digital telemetry module 
(mounted in detector cap, figure 8) to 
convert the detector's analog output to a 
digital word for transmission of the de- 
tector value along with a unique address 
and verification words to the system con- 
trol unit. A microcomputer system con- 
trol, a line Interface control module for 
communication to the computer through an 
industry standard protocol (RS232C), a 
disk drive to store the control program 
and detector output records, a color vid- 
eo display with graphics to highlight 
alarms , and a printer to provide hard 
copy of alarm and fault messages are also 
provided. 

The detectors are linked to the system 
control by a single-pair closed loop 
telemetry circuit. Connecting outsta- 
tions in a closed loop minimizes cable 
costs and installation time and provides 
a redundant signal path for uninterrupted 
signal transmission in case of a broken 
telemetry line. 

The system was completely Installed by 
a 4-person crew over a 1-week period 
(fig. 9). Minor debugging was required 
following installation because of prob- 
lems with several telemetry modules, how- 
ever, the necessary repairs were effected 
on-site during the week following the 
Installation. 



The system operated for approximately 
1 yr with a simplified control program 
while the final version of the control 
software was developed and debugged. 
During this period, system operation was 
limited to a video display of real-time 
detector outputs and an audible alarm and 
printout whenever any detector output ex- 
ceeded its individually programed alarm 
threshold. 

The final version of the control soft- 
ware provides, in addition, video graph- 
ics of the detector locations on color 
mine maps (fig. 10), simple instructions 
and three-key coded function commands 
(fig. 11), and alarm, fault, and trouble- 
shooting messages. 




FIGURE 7. - Mineworthy ionization-type com- 
bustion particle (smoke) detector. 




FIGURE 8. - Digital telemetry module mounted inside detector cap. 




FIGURE 9. - Installation of ionization-type combustion particle (smoke) detector at the 500-250 ramps. 




1188 te«L 

BECXIfCS _ 





FIGURE 10. - Mine mapon videodisplay showing 
location and output of detectors in {A) the collar- 
decline area, (6) the oxide extraction area, and 
(C) the sulfide extraction area. 



Following the on-screen prompts and us- 
ing the simple three-key commands, opera- 
tors can display one of three mine maps 
covering the system, change any sensor 
alarm threshold, display current alarm 
thresholds, display 72-h sensor history 
in tabular or graphic form, and manually 
control the printer. 

During the first 3 months of system 
operation (using the simplified con- 
trol program) numerous false alarms were 
issued. The detector-mounted digital 



telemetry modules, which are susceptible 
to low voltage conditions, were found 
to be the cause. Boosting line voltage 
slightly corrected the problem. 

The system has operated for approxi- 
mately 7 months with the final version 
of the control software. Minor debug- 
ging has been required; however, during 
this period, three abnormal events (smok- 
ing rubber drive pulleys on two pumps and 
a smoking electrical controls enclosure) 
were detected by the system. 



.TrrTnt 



THIS IS A MINE FIRE SEMSIIIS st^tn 
DISPLAY AND CONTROL TEII SItOKE 

SPREAD THROUGHOUT THE If I ME Vlf* 
ED I GATED TELEPWHC LIME. ftyTCI5$Tir 
ALARHS HILL INDICATE eCCESSIVE PftRTICU 
LATE LEVELS BY A STEADY TWE ftMO^SCREEK 
DISPLAY SHOUING THE HIIIE AREA AFFECTED 

6^ 5^84 18=46= 6 

IF PRINTER COPY IS 'iCEBED TYPE IK CPl 
AS FOURTH CHARACTER FOLLOHING THREE 
CHARACTER COH^AhD PRINTABLE REPORTS 



V vrf 



SENSOR ALfiR^PUlHl 



iii-m 



SEM = SU 



.-- nPi 



IDE EXT 
LFIDE EKT 



Mh' 



bNTE 



HHAHD. HIT RETURN 



FIGURE n. - Video display showing three-key function commands. 



Telemetry 
modules 



Detection 
■instruments 



Draw tube 




Exhaust fan 



Main mine ventilation 
exhaust borehole 



Guard house 

FIGURE 12. - Layout of major elements of spontaneous combustion detection system. 



10 



SPONTANEOUS COMBUSTION FIRE WARNING SYSTEM FOR A DEEP SILVER MINE 



By definition, spontaneous combustion 
is the outbreak of fire in combustible 
material that occurs without application 
of direct flame and is usually caused by 
slow oxidation processes under conditions 
restricting the dissipation of heat. 
Historically, several disastrous noncoal 
mine fires have been attributed to the 
spontaneous combustion of wood or of wood 
and ores in remote, inactive or sealed 
areas of mines (7). Overall, spontaneous 
combustion accounts for only about 2 pet 
of all underground metal and nonmetal 
mine fires. However, the potential seri- 
ousness of spontaneous combustion fires 
is understated by this statistic. Fire 
fighting, mine rescue and recovery, and 
related operations are complicated by the 
difficult access to the remote areas 
where spontaneous combustion fires occur. 
Since 1950, over half of all underground 
metal and nonmetal fires lasting longer 
than 24 h were caused by spontaneous com- 
bustion. Associated with the spontaneous 
combustion problem are still other inci- 
dents involving ignition of sulfide 
dusts, combustion of AN-FO heated by sur- 
rounding hot ground surfaces, and the 
continuous heating of mines utilizing 
backfill with a high sulfide content. 

Recognizing the hazard of spontaneous 
combustion, the Bureau initiated a re- 
search program in 1978 to develop a 
spontaneous combustion fire warning sys- 
tem for deep metal mines (8^). The pro- 
gram began with a comprehensive search 
of the published literature for reports 
dealing with spontaneous combustion in 
mines. In addition, industry experts 
were contacted in an effort to acquire 
relevant unpublished research findings. 
This data base, together with the re- 
sults of a series of laboratory experi- 
ments of the spontaneous heating char- 
acteristics of various sulfide ores, 
timber, and other mine combustibles, 
supported the development of a conceptual 
design for a spontaneous combustion fire 
warning system. The studies indicated 
that the earliest warning of a spontane- 
ous heating event could be achieved by 
sensing for long-term trends in the lev- 
els of carbon monoxide, carbon dioxide, 
sulfur dioxide, oxygen, and temperature. 



A prototype system comprising appropri- 
ate detection instruments (details in ap- 
pendix), telemetry, and recorders was 
designed, fabricated, laboratory tested, 
and in-mine field tested at two mines. 

Initial in-mine testing was conducted 
in 1979 in an underground copper mine in 
Arizona. Results of this phase of the 
in-mine testing program are described in 
reference 8. Follow-on testing of a 
slightly modified system (the Arizona 
studies indicated that sulfur dioxide de- 
tection could be eliminated) was initi- 
ated in 1981 at an underground silver 
mine in Idaho. 

This system was installed on surface in 
an emergency escape hoist house at the 
collar of the mine's main ventilation ex- 
haust borehole (fig. 12). The detection 




FIGURE 13. - Spontaneous fire warning system 
detection instruments. 



11 



instruments, housed in a corrosion- 
resistant fiberglass enclosure (fig. 13), 
were supplied mine air through a draw 
tube linking the enclosure with the ven- 
tilation fan cowling (fig. 14). The sys- 
tem was linked by hard wire to strip 
charts in a guard house approximately 
2,000 ft from the borehole (fig. 15). 

Signal transmission is provided by a 
mineworthy telemetry system. Telemetry 
interface modules are located at each end 
of the telemetry line; i.e., in the 
emergency escape hoist house (fig. 16) 
and the guard house (fig. 17). The sys- 
tem accepts 1- to 0-V inputs for cur- 
rent loops from the detectors. Each wire 
pair can accommodate from 1 to 48 chan- 
nels. Operating on a balanced line prin- 
ciple, and incorporating special line 
filters and protection networks, the sys- 
tem is noise immune and interference 
free. 

The oxygen analyzer used in the earlier 
test program experienced excessive drift. 



Consequently, it was replaced by a simi- 
lar unit from a different vendor. How- 
ever, this detector also suffered exces- 
sive drift and was removed approximately 
1 week after installation. A second 
electrochemical cell carbon monoxide de- 
tector was later installed and connected 
to the former oxygen analyzer's telemetry 
channel. This redundancy provided an 
opportunity to observe tracking between 
the two carbon monoxide detectors. 

After 12 months of system operation, 
the remaining detectors and telemetry 
system were functioning properly. Chart 
recordings indicated up to 15-ppm excur- 
sions in carbon monoxide values following 
end-of-shift production blasts (fig. 18). 
The two CO detectors track very closely, 
both at low levels and following the pro- 
duction blasts (the traces are slightly 
offset to facilitate data analysis). 
These readings have been validated by 
analyses of air samples collected at the 
time of the CO readings. 




FIGURE 14. - Draw tube supplying mine air from fan cowling to detection instruments. 



12 



COMPUTERIZED FIRE DETECTION SYSTEM FOR AN UNDERGROUND TRONA MINE 



The trona mines of southwestern Wyoming 
are similar in layout and operation to 
deep room-and-plllar coal mines. Typi- 
cally encompassing thousands of acres, 
utilizing numerous production sections 
and miles of belt conveyor, they are so 
large that physical monitoring of all 
mine equipment and operations is not 
feasible. 

Fire detection in these and similar 
metal and nonmetal mines is very diffi- 
cult because many potential fire hazard 
areas are not under continuous, or even 
periodic, observation. The Bureau has 
developed a fire detection system for 



settings and conditions of this type and 
in-mine tests of the system were under- 
taken at a Wyoming trona mine. 

The system represents the addition of a 
fire detection capability to an existing 
computerized monitoring and control net- 
work the mine had installed previously as 
an aid to production. The system moni- 
tors and controls the conveyors, ventila- 
tion fans, mine pumps, power substations, 
production shafts and hoists, and under- 
ground bunker ore levels. Fire detection 
system costs were significantly reduced 
by taking full advantage of in-place te- 
lemetry and associated control equipment. 





FIGURE 15. - Strip chart recorder in guard shack. 



FIGURE 16. - Telemetry interface modules 
adjacent to detection instruments. 



13 




FIGURE 17. - Telemetry interface modules in guard shack. 




FIGURE 18. - Typical chart recording showing elevated CO levels following end-of-shift blasts. 



14 



The system is designed to monitor heat, 
CO gas, submicron particulates (smoke), 
and UV radiation levels (flame) at spe- 
cific locations in the mine in order to 
detect fires at the earliest possible 
time. 

The fire detection system addition con- 
sists of four components: a submaster 
station and three remote outstations in- 
terfaced to fire detectors (fig. 19). 
The three remotes. A, B, and C, are phys- 
ically attached to the existing remote 
units (fig. 20) and utilize the existing 
remote unit telemetry system to transmit 
information from the various sensors to 
the master station. The master station 
microprocessor (fig. 21) processes the 
data, initiates alarms and warnings, and 
sends the processed information to the 
submaster station to be displayed. The 
processed information is recorded in the 
form of paper hard copy on a printer for 
mine company records. 

Remote unit A is located near a belt 
motor and grease niche area. Fire haz- 
ards in this area would include various 
combustible liquids and greases stored in 
the grease niche and overheating of belt 
drive motors. Thermistor line-type heat 
sensors were mounted above the conveyor 
belt and over the belt motors to detect 
overheating (fig. 22). CO and smoke de- 
tectors were mounted downwind of the belt 



Remote 
unit A 




Remote 
unit B 




iSi 






motors (fig. 23). The grease niche area 
is being monitored by two UV detectors 
(fig. 24). 

Remote unit B is interfaced with the 
same type of sensors as remote unit A 
with the exception of the two heat sen- 
sors. Remote unit C is interfaced to 
the same types of detectors as remote 
unit A with the exception of the two UV 
detectors. 

All analog detector signals (smoke 
and CO) are converted to digital 
form by an analog-to-digital converter 
and are transmitted to the submaster 
along with the various contact clo- 
sures and dc signals already in digital 




FIGURE 19. - Layout of major elements of 
trona mine fire detection system. 



FIGURE 20. - Added remote unit A attached 
to existing remote unit. 



15 




FIGURE 21. - Master station. 



form. The analog-to-digital conversion 
of the analog signals is accomplished 
by an incremental charge balancing 
technique. 

The telemetry system within the remote 
units transmits all signals received from 
the fire detectors to the master station 
via FSK tone (frequency shift key tone 
modulation) transmission when called by 
the microprocessor located in the master 
station. 

The transmitted data are temporarily 
stored and analyzed by the control micro- 
processor located in the master station. 
The submaster station serves as an alarm 



setpoint control for the microprocessor, 
a present time status display of pro- 
cessed data, and an alarm annunciator. 
The alarm set point controls are digital 
thumb wheels that are used to set the de- 
sired alarm level for each measured vari- 
able. When the telemetered signal ex- 
ceeds the set thumb wheel alarm level, 
the microprocessor initiates an alarm at 
the submaster station, which corresponds 
to the remote unit area and type of de- 
tector experiencing an alarm. Deactiva- 
tion of the alarm is automatic when the 
telemetered variable drops below the 
setpoint level. The microprocessor also 



16 




FIGURE 22. - Thermistor line-type detection at conveyor drive 




FIGURE 23. - Carbon monoxide and smoke detection 
instruments located downstream of the conveyor drive. 



senses which contact closures are open or 
closed and transmits the proper alarm or 
normal mode to the submaster station. 

All data received by the submaster sta- 
tion from the microprocessor in the mas- 
ter are recorded by a printer on paper. 
An alarm-status logger is provided to 
record sensor status of the various re- 
mote units in the mine for the mine com- 
pany's records. The printer provides an 
easy to read formatted output. CO levels 
and smoke particle levels are recorded 
automatically at 1-h intervals for a 
period of 24 h and are summarized at mid- 
night of each day. Any alarm conditions 
will cause the printer to record data 
instantly at 10-min intervals until the 
alarm condition subsides. 

The system has operated continuously 
since its installation in 1981 without 
hardware or software failure. 



17 




FIGURE 24. - Ultraviolet flame detector in grease niche area. 



CONCLUSIONS 



The elapsed time between the onset of a 
fire and its detection is critical be- 
cause fires tend to grow in size and in- 
tensity with time. Early fire detection 
and warning permit the initiation of a 
mine's emergency plan (evacuation, fire 
fighting, etc.) while the fire is still 
small, or ideally, while it is still in 
the incipient stage. Fire detection and 



warning systems, utilizing sensitive 
heat, flame, smoke, and gas analyzers, 
provide the most rapid and reliable indi- 
cation of a developing fire. Testing of 
prototype equipment in a variety of mine 
settings has highlighted both deficien- 
cies and advantages of various detection 
instruments and telemetry systems. 



REFERENCES 



1. Riley, R. E. Lessons We Can Learn 
From the Sunshine Mine Fire. Pres. at 
Am. Min. Congr. Annu. Meeting, Denver, 
CO, Sept. 10, 1973, 14 pp.; available 
f rom W. Fomroy, BuMines, Minneapolis, MN. 

2. Baker, R. M. , J. Nagy, and L. B. 
McDonald. An Annotated Bibliography of 
Metal and Nonmetal Mine Fire Reports 
(contract J0295035, The Allen Corp. of 
America). BuMines OFR 68(1)-81, 1980, 64 
pp.; NTIS PB 81-223729. 

3. FMC Corp. Mine Shaft Fire and 
Smoke Protection System (contract 
H0242016). BuMines OFR 24-77, 1975, 407 
pp.; NTIS PB 263577. 

4. Griffin, R. E. In-Mine Evaluation 
of Underground Fire and Smoke Detectors. 
BuMines IC 8808, 1979, 25 pp. 

5. U.S. Code of Federal Regulations. 
Title 30 — Mineral Resources; Chapter I — 
Mine Safety and Health Administration, 



Dep. of Labor; Subchapter — Coal Mine 
Safety and Health; Part 75 — Mandatory 
Safety Standards — Underground Coal Mines; 
Subpart L — Fire Protection; Sec. 75.1103- 
4 — Automatic Fire Sensor and Warning De- 
vice Systems; Installation; Minimum Re- 
quirements; July 1, 1984. 

6. Litton, C. D. Product of Combus- 
tion Fire Detection in Mines. Chapter in 
Underground Metal and Nonmetal Mine Fire 
Protection. BuMines IC 8865, 1981, pp. 
28-48. 

7. Ninteman, D. J. Spontaneous Oxi- 
dation and Combustion in Sulfide Ores 
in Underground Mines. BuMines IC 8775, 
1978, 36 pp. 

8. Stevens, R. B. Improved Spontane- 
ous Combustion Protection for Underground 
Metal Mines (contract H0282002, ESD 
Corp.). BuMines OFR 79-80, 1979, 262 
pp.; NTIS PB 80-210461. 



19 



APPENDIX 



THERiMISTOR STRIP SHAFT 
FIRE DETECTOR 

The thermistor strip detection system 
selected for the salt mine shaft was the 
Alison Control A888-M106 Fire Detection 
System. 

The control unit is housed in a Nation- 
al Electrical Manufacturers Association 
(NEMA) 12 enclosure. A separate annunci- 
ator is provided in a NEMA 9 enclosure. 
The system provides two independent, ad- 
justable levels of alarm (prealarra and 
alarm) that are annunciated at both the 
control unit and annunciator. The loca- 
tion of the hotspot is also indicated in 
feet above or below ground level at the 
annunciator. 

The sensor is completely supervised. 
An abnormal condition is indicated at the 
control unit and annunciator if an open 
or short occurs anywhere along the entire 
length of the sensor. All interconnec- 
tions between the control unit and annun- 
ciator are also supervised. 

The A888-M106 system requires 115±10 V 
ac input power. The maximum power dissi- 
pation is 300 W. 

The detection cable operates on 24 V dc 
generated by an internal f erroresonant 
power supply. Should the system lose ac 
input power, the power supply is auto- 
matically disconnected and standby bat- 
teries (located at the bottom of the con- 
trol unit) are automatically switched in. 
The batteries are sufficient to power the 
system for 24 h in standby followed by 1 
h in alarm. The system contains a bat- 
tery charger that automatically maintains 
the batteries fully charged when ac power 
is present. 

The annunciator is powered from the 
control unit at 24 V dc and is serviced 
by the control units backup batteries. 

The sensor is composed of thirty 40-ft 
sections of Alison 9090-100 continuous 
thermistor cable. This cable consists of 
stainless steel tubing containing a spe- 
cially formulated ceramic thermistor 
core. A center wire is imbedded in the 
core and runs the entire length of the 
sensor. 



The sensor center-wire-to-case resist- 
ance exhibits a negative temperature 
coefficient. This means that as the 
temperature increases, the resistance 
of the sensor decreases exponentially. 
It is this decrease in resistance that 
is sensed by the alarm instrumentation. 
Table A-1 displays the temperature- 
resistance relationship for 9090-100 
series cable. It should be noted that 
the sensor will detect a high temperature 
on a short length of the cable as well as 
a lesser temperature over a longer length 
of the cable. 

TABLE A-1. - Temperature versus 
resistance for thermistor 
strip shaft fire detector 

Temperature , °F Resistance, Q 

50 1,000,599,928 

100 69,098,544 

150 7,397,155 

200 1,111,113 

250 218,002 

300 52,998 

350 15,343 

400 5,131 

450 1,935 

500 808 

550 368 

600 180 

650 94 

700 52 

750 30 

The 40-ft sensor sections are connected 
in series to form two sensor circuits 
each 600 ft (15 sections) in length. 
Each 600-ft circuit is monitored sep- 
arately. The two circuits meet at an 
elevation of 580 ft where they are ter- 
minated in a stainless steel junction 
box. Stainless steel junction boxes are 
also provided at the -1,180- and +20-ft 
elevations to terminate the other ends of 
each sensor circuit. The entire sensor 
length and all three junction boxes are 
coated with a heavy polymer jacket for 
further protection from the corrosive 
atmosphere . 



20 



The 30 detector cable sections were 
provided by the vendor in ten 120-ft 
lengths. Three 40-ft sections were fac- 
tory spliced to produce each 120-ft 
length. The factory splices were sealed 
using a heavy duty heat shrink jacket. 

The center conductor at both ends of 
each sensor circuit is connected to the 
control unit by single conductor shielded 
cable. The sensor case is connected to 
the control unit via the shield. 

Each sensor circuit is monitored by a 
detection panel and a hotspot panel. The 
upper and lower circuits share common 
prealarm, alarm, and hotspot indicators, 
making the two circuits appear as one. 

The hotspot panel provides a 5.6 V dc 
voltage clamp that determines the sensor 
center-conductor-to-case voltage at nor- 
mal ambient temperatures. Each hot-spot 
panel generates a linear to 10 V dc 
analog voltage that is indicative of the 
hotspot location for its associated 
sensor circuit. A special combining cir- 
cuit selects the output from the circuit 
that is in alarm and converts it to the 
proper analog voltage (-18 V dc to +2 V 
dc) to drive the hotspot location meter 
at the annunciator. If both circuits are 
simultaneously in alarm, the lower cir- 
cuit overrides the upper circuit. 

If a large section of sensor is heated, 
the hotspot circuitry tends to indicate 
the edge of the hotspot closest to the 
excited end of the sensor (-600-ft eleva- 
tion) . The hotspot indication for a 
growing fire will drift towards the -600- 
ft level. 

If a section of sensor is heated to the 
point where the center-conductor-to-case 
resistance of the sensor falls to the 
prealarm setting, the amber prealarm in- 
dicator is illuminated at the control 
unit and annunciator. The auxiliary pre- 
alarm relay is energized, transferring 
three customer available form C relay 
contacts. The hotspot location indicator 
at the annunciator is also energized. 
All of these response indicators reset 
automatically when the sensor resistance 
rises above the prealarm threshold. 

If the sensor is heated to a point 
where the sensor center-conductor-to-case 
resistance falls to the alarm setting. 



the red alarm indicator is illuminated at 
the control unit and annunciator. The 
alarm responses reset automatically when 
the sensor resistance rises above the 
alarm threshold. 

An abnormal condition such as an open 
or short in the detection cable, an open 
or short between the detection cable and 
the control panel, or the loss of ac or 
dc power is indicated by audible and vis- 
ual alarms at the control panel and the 
annunciator. 

SUBMICROMETER PARTICULATE 
(SMOKE) DETECTOR 

The submlcrometer particulate detector 
selected for the underground copper mine 
and underground trona mine fire detection 
systems was the Anglo American Electron- 
ics Laboratory/Wormald Electronics Becon 
MK IV ionization type combustion particle 
detector. 

Externally the detector is cylindrical 
in shape, 10 in. in total height, and 6- 
1/8 in. in diameter (refer to figures 1 
and 2 in the main text) . A cylindrical 
cap having a height of 3-1/2 in and a 
diameter of 7-1/4 in, which houses the 
power terminal connectors and test sock- 
et, is mounted at the top of the detec- 
tor, A supension eye ring is affixed to 
the top of the cap to facilitate the 
hanging of the detector in the fire haz- 
ard area. Because of the highly humid 
and corrosive underground mine environ- 
ment , the cylindrical outer casing of the 
Becon detector is made from nylon-dipped 
stainless steel to ensure detector 
longevity and to provide a radiation 
shield. 

Towards the lower end of the Becon de- 
tector are vertical rectangular ports, 
which allow the mine air to enter the 
ionization chamber. The ports in the 
stainless steel shield are internally 
overlapped by a nylon-dipped stainless 
steel baffle plate, which shields the 
areas outside the detector from direct 
radiation, reduces effects of high veloc- 
ity airflow in the ionization chamber, 
and causes a mixing of the mine air in- 
side the ionization chamber. 



21 



Internally the Becon MK IV particle de- 
tector is comprised of a shielded single 
ionization chamber, a radioactive source, 
an ion collecting electrode (grid) , and a 
current amplifier. Because of the inher- 
ent corrosive nature of the underground 
mine atmosphere, all internal components 
of the detector are made of plastic or 
are hermetically sealed. 

The radiation source, which ionizes the 
air within the ionization chamber, is a 
sealed glass vial containing 5 mCi of 
krypton 85 gas. The vial is connected to 
the grid inside the ionization chamber by 
two cable ties. 

The ionization chamber (conducting 
plastic chamber case) is constructed from 
conducting plastic and completely encir- 
cles the grid, also made of conducting 
plastic. The plastic chamber case is 
cylindrical in shape, but the circumfer- 
ence of its walls is not continuous. In- 
stead, the wall is constructed from a 
number of overlapping curved rectangular 
plates of conducting plastic. These 
plastic plates are affixed to the disk- 
shaped base of the chamber case at two 
alternating radii about the mean circum- 
ference of the chamber. The longer edges 
of the rectangular plates run parallel to 
the axis of the chamber case. This stag- 
gering of the sides of the chamber case 
wall allows mine air to enter the chamber 
and causes further baffling of the mine 
air velocity. The plastic chamber case 
of the ionization chamber acts as the 
ground electrode with respect to the 
grid, which is the negative electrode. 
Because the plastic chamber case along 
with the conducting plastic upper case 
are at ground potential they electrically 
shield the ionization chamber and all in- 
ternal electronics from electromagnetic 
radiation external to the detector. 

The hermetically sealed amplifier 
electronics and the grid are electrical- 
ly isolated from the conducting plas- 
tic cases, by a deep annular grooved 
insulator and conductive plastic guard 
ring. The annular grooves are present to 
create the longest possible leakage path 
between the grid and the case. Electri- 
cal leakage could occur if high humidity 
saturates the inside of the detector 
with moisture or if a conductive dust is 



present in the mine atmosphere and even- 
tually settles within the detector. The 
annular grooved insulator also serves the 
purpose of supporting the grid and ampli- 
fier electronics. The guard ring pre- 
vents leakage, by its connection to the 
non-inverting terminal of the operational 
amplifier. The inverting terminal of the 
operational amplifier, which is connected 
to the grid, is maintained at the same 
potential as the non-inverting terminal, 
therefore no potential difference can 
exist between the guard ring and the 
grid, which results in no current flow. 

The Becon MK IV detector is a single 
ionization chamber analog output particle 
detector. The conducting plastic chamber 
case and the grid are separated by a po- 
tential difference of approximately 10 V. 
This potential difference, with ionized 
air as the medium, produces an ionization 
chamber base current of approximately 0.5 
nA. The 5-mCi krypton 85 beta radiation 
source (half-life of 10.8 yr) is used to 
ionize inflow air. The ionization cur- 
rent across the ionization chamber is 
adjusted by varying the potential across 
the case and the grid to yield an ioniza- 
tion current level proportional to a 
-0.9-mA output current. This base ion- 
ization current level corresponds to the 
particle concentration in normal ambient 
mine air. 

The potential difference between the 
case and the grid remains essentially 
constant, however, the ionization current 
will vary depending upon the size and 
concentration of particles carried by the 
inflow air into the ionization chamber. 

A charged smoke particle is much heav- 
ier than an air molecule, therefore its 
drift velocity due to the potential be- 
tween the ionization chamber wall (case) 
and the grid is very small compared to 
the convective airflow velocity. The 
smoke particle also has a much larger 
surface area than an air molecule, which 
reduces the mean free path between colli- 
sions of the ions responsible for current 
flow, allowing for greater numbers of 
positive ion and electron recombinations. 
Because of recombination, the electron 
and ion mobility are reduced, which re- 
sults in a detectable decrease in the 
ionization current. 



22 



A reduction in the ionization current 
corresponds to a similar reduction in the 
output of current from the current ampli- 
fier. The ionization current and there- 
fore the output current are varied by 
adjusting a potentiometer that is acces- 
sible through a hole in the outer casing 
of the detector. A capacitor ensures the 
stability of the current output and sup- 
plies the necessary feedback to maintain 
correct circuit operation when connected 
to any high output load capacitance. 

In order to prevent internal leakage 
between the grid and the chamber case be- 
cause of dust accumulations, a circular 
guard ring is installed between the case 
and the grid. Any electrical leakage 
between the grid and the casing would 
result in a reduced current flow from the 
grid to the current amplifier and cause 
an erroneous output current, A potenti- 
ometer is adjusted so as to maintain an 
input offset voltage of approximately 
V. 

Because there is no potential differ- 
ence between the guard ring and the grid, 
no current can leak between the two, 
thereby the grid is electrically isolated 
from the chamber case. 

Humidity, which causes precipitation of 
moisture on the insulation material sur- 
rounding the hermetically sealed elec- 
tronics, can also cause electrical leak- 
age between the grid and the chamber 
case. However, because of the strategic 
location of the current amplifier within 
the component box, the heat generated by 
the amplifier keeps the insulator essen- 
tially dry around the grid area. Though 
other components of the detector may be 
covered with moisture, no current can 
leak across the insulator to the grid. 

The location for mounting the Becon MK 
IV detector should be near or on the 
downwind side of a potential fire hazard 
area, however the ventilation air veloc- 
ity in the chosen area should not exceed 
6 m/s . 

Input power requirements are -15 V dc, 
-5.5 mA dc. A four-conductor shielded 
cable should be used to provide for the 
input power and output signal. 

Because the Becon MK IV detector has no 
moving parts, very little maintenance is 
necessary. Periodic examination of the 



electrical cable for breaks or frays and 
calibration are all that is required. 

Because of the natural decay of the 
krypton 85 radiation source (half-life 
10.8 yr) , the output current will drop as 
the source decays with age. Therefore, 
calibration according to the previous 
section should be carried out annually to 
ensure proper and consistent operation. 
This calibration adjustment can compen- 
sate for an approximate 50 pet reduction 
in the strength of the source. Because 
of this reduction in source strength, the 
source should be replaced at some time 
approaching the half-life of the krypton 
85. 

CARBON DIOXIDE DETECTOR FOR SPONTANEOUS 
COMBUSTION FIRE WARNING 

The Anglo American Electronics Labora- 
tory Spanair analyzer was selected for 
CO2 detection in the spontaneous combus- 
tion fire warning system. The Spanair 
nondispersive infrared analyzer detects 
the attenuation of radiation due to 
molecular absorption by the sample gas. 
Variations of the basic cell will show 
gas concentrations of CO, CO2, CH2, NO2, 
or SO2. A nichrome filament pulsed at a 
specified frequency radiates broad-band 
energy. This energy passes through the 
sample gas in a reflective optical cham- 
ber, through a spectral filter, and is 
measured by a pyroelectric cell photo- 
detector. The electrical signal output 
is inversely proportional to the gas con- 
centration. Selectivity to the sample 
gas is determined by the band-pass spec- 
tral filter. 

Both analyzer head and power supply are 
mounted in a 14- by 18-in fiberglass en- 
closure designed for underground instal- 
lation. A dust filter is fitted to the 
sample plenum. No pump or thermal con- 
trols of major importance are required. 
Electrical connection is made in a junc- 
tion box partitioned from the analyzer. 
Input power is 110 V ac , 50 to 60 c/s; 
output is to 1 V dc analog, with system 
failure indicated by a 0-V output signal. 
The output signal decreases logarithmic- 
ally with increasing gas concentration. 
Although determination of actual gas con- 
centrations require conversion of the 



23 



voltage via a calibration curve (provided 
with each unit) , unusual excursions from 
normal levels are readily apparent on 
strip charts and can trigger alarms. 

In operation, the Spanair analyzer CO2 
signal shows a constant level of about 
330 ppm, which is the concentration of 
CO2 in normal atmospheric air. As incip- 
ient heating occurs in combustible mate- 
rial, large volumes of CO2 will be given 
off well before pyrolysis begins. The 
system will report these changes as a 
gradual increase in CO2. 

A receiver (surface unit) processes the 
analyzer output into alarm levels. The 
chart records input, voltmeter, and sys- 
tem failure signals. For long distance 
data transmission, a frequency-division 
multiplex telemetry system is utilized. 
Several remote analyzer heads can com- 
municate over one balanced transmisison 
line (two wires and suitable ground). 

The only difficulties anticipated were 
a lack of published performance specifi- 
cations and a lack of repair parts or 
maintenance service available from the 
Republic of South Africa. However, a 
competent technician can maintain the 
electronic circuitry and analyzer head 
with use of the furnished manual. The 
only anticipated maintenance consists 
of periodic cleaning of the particulate 
filter if the environment is dusty. Mir- 
rors should be cleaned every few years to 
maintain a strong signal. Calibration 
requires an output adjustment to 1.0 V 
during nitrogen purge. 

Mine fire detection systems are expect- 
ed to operate under conditions that would 
normally disable laboratory instruments. 
Thus, performance data obtained under 
stable laboratory conditions do not fully 
predict performance expected for a mine 
where conditions are harsh and unstable. 
Laboratory tests were conducted to deter- 
mine the degree to which the instrument 
is immune to such harsh and unstable con- 
ditions. Conditions that are expected 
underground and that are reproducible to 
a certain degree in the laboratory in- 
clude the following: 

1. Line voltage variation between 90 
and 140 V ac. 

2. Blackouts for long time periods. 



3. Changes in temperature between 10° 
and 40° C. 

4. Changes in ambient moisture level 
between 20 and 95 pet relative humidity. 

The analyzer displayed a slight in- 
crease in sensitivity to CO2 concentra- 
tions at temperature extremes, however, 
the problem is considered to be minor. 
The analyzer is not sensitive to changes 
in relative humidity or line voltages. 
Following power interruption, the instru- 
ment restabilizes within 1 minute after 
power is restored. 

OVERHEAT DETECTION FOR CONVEYOR DRIVE 

Overheat detection for the conveyor 
drives at the trona mine was provided 
with Edison Electronics mpdel 377 control 
and model B fire detection (thermistor) 
cable. The 377 control is 2-1/2 by 1-3/4 
in. in size; is wired through an eight- 
pin connector to power (24 V dc); con- 
tains a detection cable, audio alarm, and 
lights; and is mounted to electrical ter- 
minals inside a 6- by 8-in corrosion re- 
sistant box. The model B fire detection 
cable is a thermistor; that is, a temper- 
ature sensitive resistor. The model B 
cable is tubular, 0.070 in. in total 
diameter, with a 0.020-in-diam iron wire 
center conductor imbedded in a 0.010-in- 
thick layer of metal oxide semiconducting 
material. It is 20 ft long and operates 
within the temperature range of -40° to 
2,000° F. 

The model B fire detection cable is 
similar to the cable used in the salt 
mine shaft. It is constructed with a 
metallic outer sheath and a metal wire as 
the center conductor. They are elec- 
trically isolated from each other by 
a cylindrical semiconductor layer. The 
thermistor's resistance between conduc- 
tors is depicted by a negative tempera- 
ture coefficient with a drop in resist- 
ance that is nearly exponential with a 
linear increase in temperature. The rate 
at which the resistance drops and the 
temperature at which it drops can be al- 
tered by varying the type and quality of 
the semiconductor material. 

The semiconductor material used in the 
Edison model B fire detection cable has a 



24 



conductivity between that of a metal and 
that of an insulator. Within a semicon- 
ductor there exists three discrete energy 
levels that electrons may cross or occu- 
py; the valence band, the energy gap, and 
the conduction band. If there are unoc- 
cupied high energy levels within the 
valence band or if the valence band flows 
smoothly into the conduction band, addi- 
tional kinetic energy can be given to the 
valence electrons by an applied electric 
field, resulting in conduction such as in 
a metal. However, if the valence band of 
the material is completely full and there 
exists a large energy gap, such as 6 eV, 
between the valence band and the conduc- 
tion band, the material acts as an 
insulator. 

The semiconductor material has a full 
valance band, an essentially empty con- 
duction band, an energy gap of approxi- 
mately 1 eV, and behaves as an insulator 
at room temperature. Unlike an insula- 
tor, the semiconductor, when heated, can 
gain enough thermal energy from, its sur- 
roundings to allocate electrons from the 
valence band to the conduction band. The 
semiconductor's conductivity increases 
with temperature as more electrons are 
elevated to the conduction band. Elec- 
trons in the conduction band and hole, 
vacant spots in the crystal lattice of 
the valence band, are free to move under 
the influence of an electric field. 

The resistance is measured from the 
center conductor through the temperature- 
sensitive semiconductor material to the 
outer conductor. The resistance is 
equivalent to that of an infinite number 
of resistors connected in parallel. The 
Edison 377 control senses the drop in re- 
sistance when the model B fire detection 
cable is heated, by a proportional drop 
in the potential difference across two 
conductors of the cable. If the total 
resistance of the cable is between 38 and 
315 Qy the control senses the voltage 
proportional to the resistance and acti- 
vates the fire alarm via two transistors 
that are turned on by an operational am- 
plifier. If on the other hand the re- 
sistance between the two conductors of 
the thermistor is less than 38 Q, the 



control senses a lower porportional volt- 
age through a second operational ampli- 
fier. This operational amplifier turns 
on several transistors that activate the 
cable fault light and lock out the alarm 
circuit. 

ULTRAVIOLET FLAME DETECTION 

The DetTronics U7602 ultraviolet (UV) 
flame detector was selected for fire de- 
tection in the grease niches at the trona 
mine. This detector responds to the 
wavelengths of light given off by a fire 
in the UV range of 1,850 to 2,450 A. The 
electronics are housed in an explosion- 
proof enclosure 8.91 in. in total length, 
constructed of two screw-together coaxial 
cylinders — one of 4.84-in length and 2.3- 
in diam, and the other of 4.07-in length 
and 3.25-in diam. The UV viewing area is 
a 90° cone. Input voltage is 120 V ac 
with a maximum power consumption of 3.0 
W. Two digital alarm modes are provided: 
one closed relay for fire alarm and one 
open relay for dirty lens alert. 

The DetTronics UV detector utilizes a 
Geiger-Mueller type tube to sense UV 
radiation emitted from a fire. A typical 
Geiger-Mueller tube is constructed with a 
wire anode, which operates between 160 
and 250 V above the cathode. The tube is 
sealed from the air and filled with an 
inert gas such as argon or helium. Light 
can enter through only one end of the 
tube; all other sides are optically 
isolated. 

When UV light passes into the tube, 
electrons are knocked off the gas atoms 
and ions are created. The electrons are 
accelerated to the anode because of the 
applied electric field and in turn knock 
off more electrons from the gas atoms, 
resulting in an avalanche effect. The 
ions move to the cathode and electrons to 
the anode causing the tube to conduct. 
However, when all the possible ions and 
electrons have been attracted to their 
respective electrodes, conduction ceases. 
Therefore a quenching circuit is neces- 
sary to allow the positive ions and elec- 
trons to recombine and reactivate the 
tube. 



25 



When the tube conducts it draws down 
the voltage across a capacitor. The ex- 
tinguish voltage on the detector is in 
the region of 160 V, a level at which the 
ionization processes that support the 
discharge can no longer be maintained. 
At this point, the tube will stop con- 
ducting and the capacitor will recharge 
through a resistor that is a current lim- 
iting resistor. As the capacitor re- 
charges, it will reach a voltage level in 
the vicinity of 250 V, which is the nor- 
mal striking or starting voltage of the 
tube. If UV radiation of sufficient in- 
tensity is present at this moment, the 
tube will fire again, and this process 
will be repeated over and over as long as 
radiation is present. The more intense 
the radiation the more frequent the dis- 
charge rate of the detector. 

The fire warning relay is closed when 
25 or more discharges occur per second. 

The DetTronics U7602 detector is also 
equipped with an UV test lamp that mon- 
itors the integrity of the optical lens 
and deenergizes a relay when the surfaces 
become obstructed with oil, dirt, or 
dust. The UV test lamp emits UV radia- 
tion that passes through the lens, re- 
flects off a beveled reflecting ring mir- 
ror, passes back through the lens and 
into the tube. 

CARBON MONOXIDE DETECTION 

Carbon monoxide detection for the spon- 
taneous combustion fire warning system 
and the trona mine fire detection system 
was provided by the Energetic Sciences 
Ecolyzer 4000 and the MSA 571. 

Both the MSA 571 and the Ecolyzer 4000 
are CO detectors that utilize the elec- 
trochemical properties of a fuel cell to 
sense CO. Input power to both of these 
detectors is 120 V ac. The electrochemi- 
cal sensor is constructed of three 



electrodes — the sensing electrode, the 
reference electrode, and the counter 
electrode — all suspended in an acid solu- 
tion. The materials to be chemically 
reacted are CO and oxygen gases from the 
mine's ambient air. These gases diffuse 
into the acid (or in the case of the 
Ecolyzer 4000 are pumped into the fuel 
cell by an air pump) solution and ionize. 
Refer to the following half reactions: 

2 (CO + H2O) = 2CO2 + ^^'^ + ^e- 

O2 + 4H"^ + 4e- = 2H2O, 
2C0 +02= 2CO2 

The CO is electrochemically oxidized 
at the sensing electrode while oxygen 
reduction occurs at the counter elec- 
trode. The ion concentration in the acid 
solution because of the dissolved gases 
is proportional to the concentration of 
CO in the air; likewise, the current 
flow through the cell is proportional to 
the ion concentration in the solution. 
Therefore, the current flow through the 
cell is proportional to the CO content of 
the air. This current flow is then am- 
plified and compensated for temperature 
before it is sent to the sensor control. 

The MSA 571 and Ecolyzer 4000 CO detec- 
tors are very similar in their function. 
Their input amplifiers generate a 1-V 
full-scale analog signal output from the 
signal received from the sensor cells. 
The input amplifier drives a meter on the 
detector's front panel and also provides 
a 0- to 1-V output proportional to the CO 
concentration. Operational amplifiers 
used as voltage comparators monitor the 
output voltage of the input amplifier. 
When this output voltage reaches a level 
proportional to 20 ppm, the warning relay 
activates; at a level proportional to 50 
ppm, the alarm relay activates. 



i-U.S. CPO. 1985-50SO19/20,073 



INT.-BU.OF MINES, PGH., PA. 28029 



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